1. Field of the Invention
The present invention relates to a baculovirus expression vector and method therewith for generating immunogenicity in a host, particularly to a baculovirus expression vector simultaneously displaying and expressing influenza surface protein and method therewith for generating immunogenicity in a host.
2. Description of the Prior Art
Influenza viruses, type A members of the Orthomyxoviridae family, have eight negative sense RNA segments encoding 10 proteins. Among these genes, the hemagglutinin (HA) and neuraminidase (NA) genes encode virulence-associated surface glycoproteins, are responsible for attachment of virus to terminal sialic acid residues on host cell receptors, and mediate fusion between viral and cellular membranes and detachment from infected cells. To date, viruses of 16 HA and 9 NA subtypes have been identified in avian species. Among the 16 HA subtypes, the highly pathogenic (HP) phenotypes are only associated with some strains of the H5 or H7 HA subtype.
Inactivated Vaccine
Because of high pathogenicity and high mortality of the new H5N1 avian influenza virus, novel vaccines must be developed to control and prevent its infection. Trivalent inactivated vaccine has now been used as the standard influenza vaccine and contains HA and NA of the present epidemic strain (usually two strains of influenza A and one strain of influenza B). The inactivated vaccine is prepared by inactivating the influenza virus produced in the chicken embryonated eggs with chemical reagents and extracting the necessary antigen protein for immunization by intramuscular injection. This method has been practiced for over 50 years and is still the mainstream manufacturing method of influenza vaccine for now. However, chicken embryonated eggs have some drawbacks for producing influenza vaccine. First, this method may not be used for manufacturing vaccine of high pathogenicity strains (e.g. H5N1) because they may be lethal to the eggs. The product manufactured by the method may also be unsuitable for those allergic to eggs. In addition, the inactivate vaccine has some drawbacks: (1) It induces effective neutralizing antibodies but elicits less potent cellular immune response such as cytotoxic T lymphocytes (CTL) that are required to eliminate infected cells, and hence confers less immune protection against virus and parasitized bacteria or parasites; (2) It fails to increase the generation of memory T cells and may require multiple booster injections to achieve vaccine efficacy.
Attenuated Vaccine
Attenuated vaccine, by definition, is attenuated for virus virulence but is still capable of transient growth and proliferation in vivo. The prolonged existence of attenuated vaccine in the host theoretically increases the possibility for the immune system to recognize the antigen to increase immunity and the generation of memory immune cells. The endogenous antigen expression in the host may also be an advantage for effective elicitation of cytotoxic T cells.
However, the attenuated vaccine has some drawbacks including:                (1) It is live and may revive, raising safety concerns.        (2) It may cause risks of infection for those having incompetent immune systems.        (3) It has to be produced at facilities with more stringent biosafety regulations, leading to more complicated procedures and higher cost.        
To solve the above-mentioned problems, US Patent application No. 20080003203 disclosed a pseudotyped baculovirus vector which displays HA on its membrane to generate host immunogenicity against HA and thus may be developed as a vaccine for eliciting antibody reaction to neutralize influenza virus. However, the pseudotyped baculovirus vector may elicit only humoral immune responses, but fails to trigger long-lasting cellular immune response.
Therefore, it is now a current goal to develop a novel vaccine that elicits not only the humoral immune response but also the long-acting cellular immune response.